When an air propeller is itself driven and is used as the propulsion system for a watercraft, helicopter, or land vehicle it is a so-called negative propeller. In such usage it is converting the mechanical energy applied as torque about an axis into relative axial thrust between the propeller and the fluid mass surrounding it. When used to drive a generator, as for example in a system for exploiting wind energy, it is termed a positive propeller and in effect converts the axial thrust of the surrounding fluid mass into rotation of the propeller shaft.
Negative propellers are particularly suitable for moving watercraft on small, shallow inland waterways, rivers, canals and the like, since these channels are usually so shallow that use of a standard submerged propeller or paddlewheel would be impossible. In addition such small-draft vehicles create little wake and therefore do little damage to the banks of the waterway. Thus large areas of economic importance, which due to reduced water depth of the rivers over stretches of many thousand of kilometers are not normally navigable or are navigable only by uneconomical screw- or wheel watercraft, become passable to watercraft. Furthermore such a negative propeller is the ideal drive system for a slow land vehicle which must move over terrain where traction may be very low. Positive propellers are used to drive generators intended to overcome the energy shortage.
Most current propeller work is done in airplane propellers, as such must operate with the greatest possible efficiency to economize fuel. The quality coefficient is not important for such airplane usage. When such propellers are used to drive either as positive propellers to drive a generator continuously or as negative propellers to move a landcraft or watercraft, the slip must frequently be much more than 60%, that is the relative displacement of the propeller and the fluid mass it is in will only be six-tenths that it would be if the propeller operated with no slip like a screw in a piece of wood. In this case the figure of merit or quality coefficient must be as great as possible, and the efficiency of the propeller is of secondary importance. This is also true in a helicopter where 100% slip is required for hovering, even if some efficiency is needed when the propeller is tipped to move the vehicle horizontally.
Since, however, propellers are invariably designed with efficiency as the primary consideration, propellers are not generally used in these high-slip applications like moving land and watercraft at slow but varying speed against varying resistance and driving a generator continuously but at very low speed from a propeller subject to winds of widely varying speeds.